1. Field of the Invention
This invention relates to voltage regulation of a pulse-charged direct current power supply subject to variable loading. In particular, the invention relates to an apparatus for use in regulating DC plate voltage of a cathode ray tube (CRT) or the like under rapidly changing current loads.
A retrace or flyback circuit employing a step-up transformer is typically used for providing high direct current voltage of relatively low current to the plate (or anode) circuit of a cathode ray tube, a laser, a microwave tube or the like. In such a circuit, an output diode rectifies AC from the secondary winding of step-up transformer, often call the flyback transformer, which is then used to charge an output or funnel capacitor in which charge is stored for use in supplying the DC current requirements of the load. The output capacitor is charged by a DC pulse generally having a duty cycle of less than 50% and typically on the order of 20%, corresponding to the retrace time of a raster-scan cathode ray tube system or the flyback time in a pulsed power supply.
The retrace or flyback time is the period during which there is maximum current into and out of the primary winding of the step-up transformer. A characteristic abrupt reversal of current creates a voltage pulse across a retrace capacitor in the primary circuit. The pulse across the retrace capacitor appears as a pulse across the primary winding of the step-up transformer. The retrace time is generally a fixed parameter.
The average voltage level across the output capacitor is determined by the net charge maintained in the capacitor, which is the difference between the input time-current product supplied by the retrace pulse and the output time-current product drawn off by the load. The loading of the secondary circuit is reflected to the primary and appears in the form of peak-to-peak primary current, which either leads or trails the peak-to-peak primary voltage, depending upon the impedance characteristics of the circuit. In some applications, the primary and secondary circuits may be designed to take advantage of the resonant characteristics of the AC circuit components.
An increase in the output loading, that is, an increase in the current drain, is reflected as an increase in the loading demand on the primary circuit. Consequently at least a temporary decrease in the output voltage follows a sudden change in the current load.
Inherent compensation is generally not satisfactory. For example, a change of current from 150 micro Ampere to 1150 micro Ampere in a 20,000 volt secondary voltage may cause as much as a 2,000 volt drop in the output voltage for a typical-sized output capacitor. A decrease in output voltage is highly undesirable. In a cathode ray tube, for example, the electrons emitted by the cathode are not accelerated as rapidly at lower voltages, and as a result the electrons are deflected to a greater degree along their path of travel. This undesirable condition is known as blooming.
What is needed is a mechanism for accurately regulating the voltage of a high voltage DC power to minimize effects such as blooming.
2. Description of the Prior Art
Various voltage regulation schemes are known. Generally, voltage regulation requires the use of active components employing complex feedback circuitry to monitor and accurately control the level of the retrace pulse. Such schemes may involve the use of a sensing resistor to generate a feedback signal. Such prior art schemes are often expensive. Only the most expensive and complex schemes known provide accurate and reliable voltage regulation. There is a need to provide for accurate and reliable voltage regulation which is also inexpensive. Inexpensive and inherently reliable voltage regulators are extremely important in cost sensitive applications such as consumer-oriented products, where sophisticated schemes are simply not practical because of cost considerations.